A review of soil workability models and their limitations in temperate regions

Abstract. Soil workability and trafficability are frequently estimated from meteorological data. This paper discusses the importance of soil workability for crop production, and reviews the limitations of various models, especially their applicability for predicting the effects of climate change. Some models calculate the number of spring or autumn work-days by combining meteorological and soil-related factors. Most of these determine, either empirically or deterministically, daily soil moisture budgets from which workability is inferred. A shortcoming of this approach is that it requires critical soil moisture contents or potentials. Also there are problems with the persistence of water in the profile and with the allocation of rainfall events to specific days. Determining work-days over seasonal periods avoids these limitations, but this approach is unsuitable for short-term planning purposes. The value of workability models is difficult to gauge as they often apply to specific soil and climatic conditions. This limitation will increase under a changing climate. For example, soil properties, previously considered as intrinsic, may alter in a manner that invalidates some models. More work on the soil processes controlling workability is needed, and on how these will be affected by a changing climate.     

花岗片麻岩低山丘陵区不同植物径流小区水土流失规律研究

以山东省临朐县辛庄试验站多年的观测资料为依据,分析了径流小区内不同林草植被与径流和侵蚀的关系,研究表明,在相同坡度、坡长条件下,4种草本植物的径流小区的汛期径流量、侵蚀量随降雨量的增大而增加,4种草本种植的径流小区的汛期径流量与侵蚀量呈Y=aX＋b线性关系,花生径流小区侵蚀量的增长率分别是自然草地、沙打旺和草木樨径流小区的1．72倍、2．34倍和3．54倍。4年生灌木林地的径流量和侵蚀量明显小于对照荒坡,土壤物理性状、土壤含水量、土壤饱和贮水量和土壤渗透速度以葛藤林地最好,其次是桑树、胡枝子、花椒、李树林地,花生小区最差;分析降雨量、降雨强度对灌木径流小区径流量和侵蚀量的影响,表明不同灌木林的蓄水保土效益以葛藤最好,其次为桑树、胡枝子、花椒、李树。     

重塑黏土圆锥指数和抗剪强度的关系

为了定量评价轮式车辆在软黏土条件的附着、牵引及通过性能并建立软黏土的圆锥指数与抗剪强度关系,采用非标准普式击实法制备黏土试样,分别通过微型贯入试验与直剪试验得到重塑黏土的圆锥指数与抗剪强度,发现23%含水率是二者变化趋势的分界点,重塑土样的含水率低于该值时其圆锥指数与抗剪强度都随含水率呈二次多项式关系变化,而在高于该临界含水率时二者都随含水率呈线性关系变化。同样,在23%含水率以下圆锥指数与抗剪强度线性相关,而当含水率高于临界点后该线性关系消失。抛开含水率变化的影响,临界含水率以下的重塑样圆锥指数与抗剪强度关系仍然总体呈线性相关关系。     

Predicting soil workability and fragmentation in tillage: a review

Soil workability and friability are required parameters to consider when creating suitable seedbeds for crop establishment and growth. Knowledge of soil workability is important for scheduling tillage operations and for reducing the risk of tillage‐induced structural degradation of soils. A reliable evaluation of soil workability implies a distinctive definition of the critical water content (wet and dry limits) for tillage. In this review, we provide a comprehensive assessment of the methods for determining soil workability, and the effects of soil properties and tillage systems on soil workability and fragmentation. The strengths and limitations of the different methods for evaluating the water content for soil workability, such as the plastic limit, soil water retention curve (SWRC), standard Proctor compaction test, field assessment, moisture‐pressure‐volume diagram, air permeability and drop‐shatter tests are discussed. Our review reveals that there is limited information on the dry limit and the range of water content for soil workability for different textured soils. We identify the need for further research to evaluate soil workability on undisturbed soils using a combination of SWRC and the drop‐shatter tests or tensile strength; (i) to quantify the effects of soil texture, organic matter and compaction on soil workability; and (ii) to compare soil water content for workability in the field with theoretical soil workability, thereby improving the prediction of soil workability as part of a decision support system for tillage operations.     

Water-related ecological impacts of rill erosion processes in Mediterranean-dry reclaimed slopes

Soil moisture is considered the main limiting factor governing the structure and dynamics of vegetation in drylands. Soil erosion is perceived as a critical process affecting these systems, especially when rill formation occurs, as rill networks can condition the availability and spatial distribution of soil moisture. To assess the impact of soil erosion processes on the dynamics of Mediterranean-dry reclaimed systems, during the 2005–06 hydrological year we monitored the soil moisture regime (temporal availability and spatial distribution) and the associated responses describing vegetation performance (plant water status and potential seed germination) and vegetation structure in five coal-mining reclaimed slopes subjected to different rill erosion rates (from 0 to about 70 t ha^? 1 year^? 1). Rill network development leads to increased runoff connectivity and to concentration of water flow along the channeling network. As a result, water loss from the slope system is maximized. Simultaneously, the spatial distribution of soil moisture is ruled by the pattern of geomorphic forms (rill and interrill units). The ecological consequences are led by the intensification of water stress and the occurrence of unfavorable conditions for plant recruitment and natural colonization, causing a non-linear decline of species richness and aboveground biomass at the slope scale level. When dense rill networks are developed, long-term effects of erosion result in a sharp ecosystem transition to a very simple and low productive plant community spatially organized in downward spots adjacent to the rills, where plants minimize simultaneously water stress and the mechanical disturbance associated to concentrated flows.     

Topsoil status of cohesive soils as related to trafficability

Adapting soil engaging operations to the soil moisture status may limit damage of the soil structure or degradation processes. This requires the quantification of soil trafficability in dependence on soil hydrological conditions. The aim was to test the relevance of topsoil parameters in relation to trafficability and to study their temporal variability during a springtime period.

On different sites located in the holocene regions east of Berlin, soil moisture, density and strength parameters were analysed and related to the actual score of trafficability. The results show that those parameters being most suitable to characterise the trafficability in the field such as consistence and density, are easy and reliable to estimate on the basis on framework guidelines. Vehicle mobility and the risk of topsoil and subsoil structural damage are mainly influenced by the topsoil status of the upper 10 cm, in particular of the soil surface of cohesive soils. Trafficability state may be derived at least from two or more relevant consistency and density parameters.

Trafficability states are autocorrelated in the time domain. One‐step‐ahead forecasting of trafficability is possible using autocorrelations and weather data.     

晋西黄土丘陵沟壑区坡面土壤侵蚀及预报研究

细沟间侵蚀是指坡面上细沟未形成之前或细沟形成之后细沟间的土壤侵蚀过程。细沟间侵蚀量预报是整个坡面水土流失预报的一个重要组成部分。本文以晋西黄土丘陵沟壑区的土壤侵蚀为背景,以野外及室内试验资料为基础,论述了降雨动能、土壤抗剪强度、坡度、植被、表土结皮对细沟间土壤侵蚀的影响,提出了坡面一次降雨细沟间侵蚀总量的预报模型。本模型与径流及细沟侵蚀量预报模型结合,可形成一个较为完整的坡面水蚀总量预报模型。     

坡面水蚀比尺模型室内外相似性试验研究

对野外和室内在25°陡坡、人工降雨条件下的侵蚀产沙试验进行了对比分析,结果表明:在降雨强度相同或相近时,室内外降雨侵蚀力非常接近,但是土壤可蚀性却相差4倍左右;当其它影响因素如土壤容重、土壤前期含水量等相同或相近时,仅要达到累计侵蚀产沙量的相似,室内坡面模型可以采用2∶1的几何比尺,该比尺下室内外侵蚀产沙量的误差小于10%;从侵蚀产沙速率来看,室内外试验侵蚀产沙的过程差异较大;室内外试验都是降雨强度越大,坡面产生的细沟数量越多,并且细沟的平均长度、宽度、深度也越大;但要做到室内外坡面形态发育的相似还存在较大难度。     

Performance of soil and water conservation practices in the erosion evolution process,runoff dynamics and surface roughness

Soil and water conservation practices are used widely to prevent soil erosion and protect soil and water resources, which is significant for ecological restoration and food security. However, rill evolution processes, erosion and deposition characteristics and critical hydrodynamic parameters need more research. In order to investigate the effect of soil and water conservation practices on soil erosion dynamics, simulated rainfall experiments were undertaken in a laboratory on 15° loess slopes with engineering measures (fish-scale pits, FSPs), tillage measures (artificial digging, AD; contour ploughing, CP) and bare slope (CK). The results showed that: (1) during rill erosion, hillslopes with FSPs, CP and AD were more likely to develop wide and shallow rills, while a bare slope (CK) was more likely to develop narrow and deep rills. At the end of the experiment (cumulative rainfall was about 150 mm), headward retreat erosion dominated the AD slope (maximum rill length: 3.27 m), side-wall expansion erosion dominated the CP slope (maximum rill width: 0.522 m) and bed incision erosion dominated the CK (maximum rill depth: 0.09 m); (2) soil and water conservation practices reduced surface erosion and sediment transport and runoff velocity. However, the positive effects disappeared when rainfall amounts exceeded 82.5, 105 and 127.5 mm for FSPs, CP and AD, respectively; (3) for runoff kinetic energy and runoff shear strength of 3 J and 1.5 N/m², respectively, soil and water conservation measures had greater anti-erosion abilities than CK; (4) as rainfall duration increased, surface roughness, runoff rate and sediment concentration increased on the CK and FSP treatments, but decreased on the CP and AD treatments. This study has important implications for managing different soil and water conservation measures based on rainfall conditions and offers a deeper understanding of soil erosion processes.     

Tillage, mineralization and leaching: phosphate

Phosphate is usually the limiting nutrient for the formation of algal blooms in freshwater bodies, so tillage practices must minimize phosphate losses by leaching and surface run-off from cultivated land. Mineral soils usually contain 30–70% of their phosphate in organic forms, and both organic and inorganic phosphate are found in the soil solution. Some organic phosphates, notably the inositol phosphates, are as strongly sorbed by soil as inorganic phosphates, and this decreases their susceptibility to mineralization. The strength with which both categories are sorbed lessens the risk of their being leached as solutes but makes it more likely that they will be carried from the soil on colloidal or particulate matter, and the greatest losses of phosphate from the soil usually occur by surface run-off and erosion. Recent studies at Rothamsted have, however, shown substantial concentrations of phosphate in drainage from plots that have long received more phosphate as fertilizer than is removed in crops. These losses probably occurred because preferential water flow carried the phosphate rapidly from the surface soil to the field drains. For lessening losses of phosphate by leaching and run-off, the prime requirement of tillage is that it should encourage flows of water through the soil that help it to retain phosphate. Primary and secondary tillage should ensure that the surface roughness and porosity of the top-soil encourage the flow of water into the soil matrix where it will move relatively slowly and allow phosphate to be sorbed, thereby avoiding problems from run-off and preferential flow. Inversion tillage can be useful for lessening the loss of phosphate by run-off and erosion. Secondary tillage could be used to decrease the size of the aggregates and increase the surface area for sorption. Although tillage will increase the mineralization of organic phosphate, pulses of mineralization are unlikely to be so rapid or to lead to such large losses as with nitrate. The strength with which phosphate is sorbed also lessens the problem. As with nitrate, the key to managing phosphate is basically good husbandry.     

The use of a new portable erosion measuring device for assessing the erodibility of the surface layers of flood embankments

When a river overtops its flood banks, water running down the landward side of the bank can rapidly erode the soil surface and scour the bank, sometimes leading to breaches and collapse. A covering of living vegetation, particularly grass, can reduce this risk of water erosion. As part of a project to assess the effectiveness of different management regimes on bank vegetation cover, direct measurements were required of the erodibility of the soil surface. A portable erosion measurement device (EMD) was developed by LAB Coastal with the support of the Environment Agency. This could direct water flowing at known velocities across areas of the flood banks, and it was used to test directly the erosion resistance of vegetated grass banks at three sites. The EMD gave a direct measure of the erodibility of a small sample of flood bank. While measurements of soil strength and assessments of vegetation cover were useful, they did not always correctly characterize the stability of the bank surface as measured directly by the EMD. The EMD and its use are described and the results obtained are discussed. The results indicate that flood banks need to be mown at least once a year to make them less vulnerable to erosion.     

风蚀过程中翻耕农田土壤抗剪强度变化

为探究土壤风蚀过程中土壤抗剪强度的变化,以河北坝上干扰破坏地表及未干扰破坏地表的典型翻耕农田为研究对象,采用土壤抗剪强度野外原位直剪试验测定了土壤抗剪强度在风蚀事件前后及风蚀季的变化,结合室内试验分析了土壤抗剪强度与土壤干团聚体粒度的关系。结果表明：未受到干扰破坏及受到干扰破坏2种类型农田地表土壤抗剪切强度与作用在土壤表面的法向应力呈正相关关系,土壤抗剪强度随法向应力的增大而增大;土壤抗剪强度变化与风蚀事件的发生密切相关,除2021年4月27,28日风蚀事件后表层土壤受含水率影响外,其余2次风蚀事件后土壤抗剪强度均发生明显增大,整个风蚀季的土壤抗剪强度变化也呈现出先增大再减小最后略增大的趋势;在风蚀过程中,受到干扰破坏农田地表对于土壤风蚀过程的响应更为敏感,其抗剪强度变化较未受干扰破坏地表抗剪强度变化更为剧烈;土壤抗剪强度随土壤干团聚体平均重量直径增大而增大。为减少坝上地区风蚀的危害,应合理采用保护性耕作措施,在风蚀季减少对农田表层的干扰。     

Using simulation to define moisture availability and trafficability for a heavy clay soil in the Netherlands

Computer simulation was used in assessing moisture availability and trafficabillity of a heavy clay soil in grass. Calculations were made of the soil-moisture regime over a period of 30 years using measured meteorological and water-table data. Soil-physical input data consisted of moisture-retention and hydraulic-conductivity data, which were determined on large, undisturbed soil columns. The effect of vertical and horizontal soil cracks was assessed by independent quantitative measurements. Results of the calculations were expressed in terms of the probability of occurrence of: (1) precipitation deficits, (2) moisture deficits considering the calculated soil-moisture regime, and (3) adequate trafficability. Results referred to successive 10-day periods during the year. The magnitude of the moisture deficit was also expressed in terms of its yearly probability. Simulation models are essential tools for a flexible characterization of land qualities in terms of probability estimates which are useful for planning purposes.     

Experimental effects of antecedent moisture and soil strength on rainwash erosion of two luvisols,Ontario

The effect of antecedent moisture on rainfall erosion was evaluated in a series of field experiments by applying artificial rainfall at 50 mm h⁻¹ (0.189 J m⁻² s⁻¹), to two Gray-Brown Luvisols on plots of 7.8 m². The measured soil loss is significantly related to antecedent moisture. Extrapolations from the data indicate that soil loss may differ by as much as 800 times if the full range of antecedent soil moisture is considered. For the Guelph silt loam, the maximum soil moisture effect was observed at 20 min after the beginning of simulated rainfall.Torsional shear strength of remoulded samples at field bulk density was determined by using a mechanized torvane. Field shear strength was then estimated on the basis of the shear strength-soil moisture relationship established in the laboratory. Because sharp changes in both soil shear strength and soil loss were observed around the ‘vane shear’ liquid limit, we suggest that the influence of soil moisture is at least in part expressed via soil strength.     

基于三轴UU试验的土体含水率对根土复合体强度特性的影响

为研究自然边坡其强度特性随含水率的变化规律。通过设计不同体积含水率(20%,27%,35%,40%,45%)和2种含根量(RAR=0.1%,0.3%)的重塑土进行不固结不排水三轴试验,以研究含水率对土体破坏方式、抗剪强度和初始切线模量的影响。结果表明：(1)试样破坏模式多为剪切变形和剪胀变形,在含水率为20%,27%时,出现明显剪切贯穿面破裂带,其余含水率下表现为剪胀变形;(2)含水率对根土复合体抗剪强度主要体现在黏聚力上,含水率从20%升至45%时,2种含根量的根土复合体黏聚力分别降低67%,72%;含水率低于35%时,可用对数关系表示,随含水率升高,黏聚力明显降低;含水率达到35%以上时,可用线性关系表示,黏聚力下降速度减缓;含根量增加,抗剪强度提高;(3)不同含根量和围压下根土复合体初始切线模量与含水率关系可以用线性关系拟合,含水率从20%升至45%时,2种含根量的根土复合体初始切线模量平均分别降低43%和47%。综合考虑土体弹塑性和植物因素对土体位移或变形的影响,可提高边坡稳定性计算精度。研究结果可为植物边坡以及水土保持植被建设治理提供科学参考,丰富根系固土领域的科学理论。     

Mechanical soil conservation with contour ridges: Cure for,or cause of,rill erosion?

Based on the results of an erosion damage assessment in Southern Zimbabwe, where mechanical conservation work has been carried out since the 1940s, this paper describes the impact of mechanical conservation systems on processes leading to rill erosion. In a study of a catchment area, it was found that influxes of water from roads and waterways as well as contour ridges that were originally designed to control rill erosion had a major role in the formation of rills. Existing rills and depressions which cause water concentration, siltation of contour drains and overflowing of contour ridges were the main factors leading to excessive rill erosion. The study showed that particularly during a highly erosive year like 1992/93, the damage due to rill erosion can be excessive, causing an abrupt degradation. It is concluded that the present conservation system is insufficient to control rill erosion effectively and is often the cause of this erosion. Effective control of rill erosion is a pre-condition for optimal implementation of land management systems such as conservation tillage. Therefore, an integrated approach to land husbandry must be developed jointly with farmers and promoted in order to improve crop production and sustainable management of natural resources. This should consider improved mechanical conservation as well as agronomic and biological soil and water conservation techniques.     

Spatial and temporal variations in resistance of loess-derived soils to ephemeral gully erosion

Planners would like a simple means of describing spatial and temporal variations in soil erodibility accurately. We have done a series of concentrated flow detachment experiments to investigate the feasibility. Four different soil horizons, typical of loess‐derived soils in Belgium, were sampled seven times during one year, so that a representative range of initial soil moisture contents was obtained. Undisturbed soil samples were subjected to five different combinations of slope gradient and concentrated flow discharge. Results showed that for a given soil horizon, variations in detachment rate could be related well to temporal variations in initial soil moisture content. For a given initial soil moisture content the ploughed topsoil horizon (A_p) and the underlying clay‐enriched horizon (B_t) had only one fifth of the erodibility of the loess horizon whether decalcified (C₁) or still calcareous (C₂). Combining knowledge on spatial distribution of soil profiles and initial soil moisture content allowed us to explain observed spatial and temporal variations in resistance to ephemeral gully erosion for soils in loess. Also, differences in ephemeral gully morphology (cross‐sections) could be explained from differences in initial soil moisture content and soil horizon. In the short term these results have important implications for spatial and temporal variations in erosion, while in the medium or long term information on spatial distribution of soil profiles is crucial when predicting the volumes and patterns of (ephemeral gully) erosion. Finally, the importance of combining the effect of water and tillage erosion with respect to soil profile evolution and consequent erosion risk is stressed.     

Rill and gully development during an extreme winter runoff event in Norway

Erosion on agricultural fields in Norway has earlier been reported to be mainly sheet and minor rill erosion. Exceptional rainfall events during January and February (230% of 30 years monthly mean) in 1990 led to extreme runoff and widespread erosion also on areas normally not considered to have a high erosion risk. This study presents the results of a field survey in three counties where rills and ephemeral gullies were measured after this event. Shifting weather conditions with freezing and thawing had given frozen subsoil. Topsoil conditions varied from ice- and snow-covered surface to thawed surface with frozen subsoil. In one of the areas with a high clay content, only the upper topsoil layer was unfrozen during this event. Extended sheet erosion and smaller rills occurred. In the southernmost locations, more of the topsoil profile was unfrozen. Combined with a high silt/sand content, more severe rilling and ephemeral gullies developed. Gullies developed down to the depth of the drainpipes in all locations. This erosion equals soil losses of more than 100 tons ha⁻¹ or 8–9 mm soil lost from the entire field area. Different mechanisms for gully development were registered. Gullies developed as enlarged rills with headward migration and sidewall sloughing. Knickpoints in the slope could also lead to gully development. Concentrated water flow entering the field from farmyards, roads, neighbouring fields and woods could be the starting point for uncontrolled rilling and ephemeral gullying. The combination of frozen subsoil, saturated soil with low strength and intense rainfall led to gully development also on areas with gentle slopes, especially sandy soils. Management practices like crop cover, tillage and lack of surface water control highly influenced the development of gullies.     

水流作用下疏松土壤材料中细沟的再生及其临界剪应力的实验研究

动态土壤侵蚀包括依赖于空间与时间的土壤剥离、细沟的形成、水流中过量泥沙的沉积、沉积泥沙中细沟的再生等物理过程。与泥沙从沉积区域再剥离相关联的细沟再生是细沟变窄加深过程中的一个重要的现象。该文介绍一种实验装置,用于测量类似于沉积土壤的疏松土壤材料细沟再生的力学参数。用一种沙壤土进行实验,测定了不同团聚体大小和坡度下细沟再生开始和停止所对应的临界水流量。计算出了与沟坡相关的临界剪切应力。这些结果有助于理解细沟的再生及其数量关系,以及临界剪切应力与坡度间的定量关系;对于发展由沉积土壤再生的细沟演化过程模型特别有用。     

估算细沟含沙水流剥蚀率的改进方法

为了得到更接近实际的细沟侵蚀模拟数据,改进了前人研究细沟含沙水流剥蚀率的方法。选取黄土高原的典型土壤(安塞黄绵土),采用12 m长土槽在5个坡度(5°,10°,15°,20°,25°)和3个流量(2、4、8 L/min)条件下进行细沟侵蚀过程模拟试验。估算各水力工况下沿细沟含沙水流剥蚀率,探究含沙量,沟长,坡度及流量对于剥蚀率的影响并验证该试验方法的准确性。结果表明:剥蚀率随含沙量的增加呈线性递减,在陡坡(15°,20°,25°)上,随细沟长度的递增呈指数下降,该变化规律在陡坡和大流量下更为显著;并与前人数据进行对比分析,相关系数为0.917,说明与前人结果吻合度高,验证了该研究试验方法的准确性。研究结果将为更好地描述黄土细沟侵蚀过程及土壤侵蚀预测预报提供参考依据。